This invention relates to a method for distributing hydraulic flow between a plurality of hydraulic actuators wherein at least one of the flow rates of the actuators is determined not by a manual control but by a valve controller.
Many construction and work vehicles typically for earth moving purposes, have many, if not all of their systems, driven by hydraulic fluid. In the case of a backhoe, for example, the engine in the vehicle not only drives the backhoe over the ground, but drives boom swing cylinders that move the backhoe arm laterally side-to-side, as well as a boom cylinder to lift and lower the boom, a dipper cylinder to lift and lower the dipper and a bucket cylinder for opening and closing the bucket. In the case of a front loader, the engine not only drives the vehicle across the ground, but also drives a hydraulic pump that is connected to one or more arm cylinders to lift and lower the two arms on which the bucket is attached and one or more tilt cylinders to tilt the front loader bucket in or out with respect to the vehicle. In the case of road graders, for example, several different hydraulic actuators are used to angle the blade with respect to the road, tilt it, and raise it and lower it. In the case of forklifts, several hydraulic actuators are used to raise and lower the forks, tilt the forks backwards and forwards so the load will be located over the vehicle or away from the vehicle, and to extend the forks (in some types of lifts) to place a packet on a high shelf without moving the vehicle itself back and forth.
In addition to these examples, one should recognize that many work vehicles also provide for auxiliary hydraulic devices to be attached and detached for use in special situations. For example, hydraulic post hole diggers which include a hydraulic motor and a rotating bit approximately eight inches (8xe2x80x3) in diameter are often attached to a front loader or a backhoe in place of the bucket. As another example, pneumatic or hydraulic pavement breakers are often mounted on the front of skid-steer loaders in place of a bucket to break up pavement. These attachments are typically separately controllable through an auxiliary hydraulic control manifold to which they are attached with quick-connects.
One of the continuing problems of work vehicles is that the market is highly competitive and they must be made to sell at a reasonable price. This always involves design and engineering trade-offs in which the designers and engineers attempt to identify the most common uses and ensure that the vehicle is able to perform those functions. The inevitable compromises typically include providing a hydraulic pump that does not have capacity sufficient to simultaneously drive every single hydraulic actuator and motor without being overloaded. By xe2x80x9coverloadedxe2x80x9d I mean that the motor cannot provide pressurized hydraulic fluid at a sufficient flow rate to drive all the devices simultaneously. Inevitably, in almost every work vehicle, there is some point within the performance envelope in which the pump, providing as much fluid as it can, is unable to drive the actuators and motors as fast as the operator commands them.
The operator commands these various actuators or motors by either operating on/off switches, by moving a proportional control lever, rotating a potentiometer, or manipulating a one or two axis joy stick. Most commonly, two or more of these controls are provided for the operator to manipulate. Most of the controls are configured to generate a flow rate roughly proportional to the degree of deflection of the control lever. By manipulating two proportional control levers, the operator can vary the speed of two separate hydraulic actuators in order to coordinate the movement of one or more actuators at the same time. For example, an operator may extend the boom of a backhoe while simultaneously lifting the dipper and opening the bucket by manipulating two joysticks, one in each hand. This permits the operator to substantially increase the productivity he would have if he could only operate one actuator at a time. In many earth working operations or excavating operations, the operator must control at least two actuators at once in order to dig or shape a hole in the ground, for example. If an operator of a backhoe is attempting to scrape the bottom of an excavation flat, he will typically have to operate the boom, the dipper, and the bucket cylinders simultaneously. No single control can be operated to follow the contours of the ground as accurately as all three together.
It is in these situations where the limitations of the pump are most apparent. An operator who is trying to simultaneously swing a boom while raising the boom, extending the dipper and opening the bucket may find that there is insufficient hydraulic fluid and one or more of the hydraulic cylinders may suddenly cease moving. If the operator has been manipulating in his various hand controls and levers in order to achieve a smooth coordinated movement, the sudden erratic motion of one hydraulic actuator may gouge the ground improperly, making an error in excavation that he must later go back and repair.
One result of this failure of the pump to provide sufficient pressurized hydraulic fluid flow is that operators instinctively slow down whenever they operate several different actuators simultaneously. From experience, they know that something may xe2x80x9cgrind to a haltxe2x80x9d as they are trying to perform the coordinated operation. As a result, they slow the entire operation down until it is performed at a coordinated speed that they are reasonably assured will be within the flow capacity of the hydraulic pump on the vehicle. This, however, requires years of experience, and even with the experience, may cause the operator to operate well within the permissible total flow capacity of the pump, thus reducing his productivity. In other words, he may slow down unnecessarily.
In U.S. Pat. No. 4,712,376 issued to Hadank and assigned to Caterpillar Corporation at issue, one way of compensating for this problem was described. In the compensation method described in Hadank, the operator would simultaneously operate two controls moving them to positions that were roughly proportionate to the flow rate to the actuators and therefore to the speed of movement of the actuators. Rather than convert the control signals directly to a flow rate (or rather valve position) and drive the proportional control hydraulic valve to that opening, an electronic valve controller would read the signals from the two manual proportional controls (joysticks) would sum the two flow rates that were equivalent to those two positions and would determine what proportion of the total available flow from the pump those commanded flow rates (or valve openings) represented. For example, if the operator moved one control lever indicating that the hydraulic valve for that levers actuator should be open 100% and the operator moved another control lever for another actuator to a position that indicated it should also be open 100%, the control system would add these two requested flow rates or demand signals together. If the two 100% flow rates added up to 150% of the total hydraulic flow capacity of the hydraulic pump, the electronic valve controller would scale both of the signals back proportionately. In other words, since the operator was requesting for each flow controller 50% more flow than could be handled together, the electronic valve controller would send a proportionately reduced signal of 66% (instead of the 100%) to the first hydraulic valve and 66% (of the second hand control) to the second hydraulic valve. In this manner, the total flow permitted through the two proportional control valves would always be within the total flow capacity of the pump. No valve or actuator or motor would be starved of hydraulic fluid. What the operator would notice when manipulating the two hand controls was that the relative motion of each hydraulic actuator stayed the same, while the overall speed of both actuators was proportionately scaled back.
In recent years, however, electronically controlled work vehicles have become more and more commonplace. Part of this vehicle development has included the creation of several features and capabilities that were not heretofore possible. For example, many backhoes have an auxiliary hydraulic valve controller that responds to buttons and proportional control devices, such as thumb wheels on the operating levers, to permit the operator to set a predetermined auxiliary hydraulic flow rate. A typical case where this would occur would be where an operator of a backhoe wishes to spin the post hole digger at its most effective speed without having to constantly hold his hand on a proportional control lever to maintain that speed. The operator would like to vary the speed of the posthole digger at the end of the backhoe arm until it is at the optimum speed, then save that speed (e.g. valve opening/flow rate) and have the electronic controller maintain the posthole digger at that speed all the time as the operator manually moves the backhoe arm to which it is attached. As an additional complicating factor, work vehicles often coordinate the movement of several hydraulic actuators in response to the motion of a single operator device. Where the vehicle""s controller coordinates the motion of several actuators by generating a time-varying signal or signals that it applies to one or more other actuators, the system shown in Hadank will not ensure that the total flow rate is within the capacity of the hydraulic pump.
In accordance with a first embodiment of the invention, a valve control system is disclosed for a work vehicle having a plurality of actuators coupled to a plurality of mechanical devices to move the devices, the vehicle having an internal combustion engine coupled to at least one hydraulic pump such that there is a total or maximum flow rate available from the at least one pump to be provided to the actuators to move the mechanical devices, such as a motor for an implement, a hydraulic cylinder that moves the bucket, dipper, or boom in a backhoe, a cylinder that raises or lowers a fork in a fork lift, or tilts a fork in a fork lift, or extends the forks at the top of a fork lift, or cylinders for raising the arms of a front loader or tilting the front loader bucket. The system includes a couple hand controls that produce signals equivalent to the distance they are moved by the operator, a controller to which they are attached, proportional control valves that are driven by the controller in response to the hind control signals and a signal developed or derived by the controller itself, and the actuators that are moved by the valves. The controller receives the hand control signals, processes them and generates the valve signals to open the valves accordingly. If the operator and the controller have requested too much flowxe2x80x94more flow that the pump on the vehicle can providexe2x80x94the controller scales the flow to each actuator down, preferably proportionately, to insure that the flow demands as scaled are within the capacity of the pump to provide fluid. There may be some hydraulic devices, however, that need a set amount of flow and therefore should not be scaled. For these types of devices, the controller automatically provides them with their appropriate flow rate, subtracting this amount of flow off the top of the available flow, then proceeds to scale down and divide up the remaining flow among the remaining controllers.